Typically, it is desired that the transmit chain of wireless communication systems exhibit good linearity (equal gain independent of signal amplitude and frequency). Any non-linearity may distort the transmitted signals so that the performance of the receiver at the other end of the communication link is degraded.
Non-linearity can also lead to unwanted spectral emissions in neighboring frequency channels or at out-of-band frequencies that would degrade the performance of communication equipment using the affected part of the frequency spectrum.
The linearity of a power amplifier of a transmit chain is often the dominant parameter for the overall transmitter linearity. Power amplifiers of class-A or class-AB are typically used to achieve good linearity. The linearity of these types of amplifiers can be limited by the voltage swing that may develop at the internal nodes of the amplifier. The voltage swing may be limited by the available supply voltage. Thus, limiting the supply voltage of these types of amplifiers typically degrades linearity but improves power efficiency.
Two known methods of reducing power amplifier current consumption are average power tracking (APT) and envelope tracking (ET).
In a system using APT, the supply voltage of the power amplifier is adjusted according to the desired transmit power level. The supply voltage is chosen such that sufficient linearity may be provided in order to meet all performance requirements for the required output power. FIG. 1 illustrates an APT scheme. As can be seen, for a period of time 10 the power amplifier supply voltage 12 is held at a constant level which is a function of the average signal power of signal 14 across duration 10. The power amplifier supply may be generated by a switched-mode power supply so that the reduced supply voltage is translated directly into a power saving from the overall system power supply.
In ET systems, the supply voltage of the power amplifier follows the envelope of the transmission system in real-time. FIG. 2 illustrates an ET scheme. The system may be based on a mapping function that relates the instantaneous signal magnitude 16 to the required power amplifier supply voltage 12. The system may be set up in order that all performance parameters such as adjacent channel rejection are met under all conditions. In particular, the mapping function may be chosen such that power amplifier linearity is maintained for all transmit configurations. The supply voltage 12 closely follows the signal magnitude 16 as can be seen. This minimizes the power amplifier voltage headroom and therefore maximizes the saving of power.
Whilst APT and ET schemes aim to save power and provide an efficient power amplifier whilst maintaining amplifier linearity, additional transmission characteristics of both the transmission scheme as a whole and also the signal to be transmitted are not taken into account.
Thus an improved control of amplifier power supply is desired that takes into account the signal to be transmitted and the transmission scheme in a more detailed manner whilst retaining amplifier linearity and improving efficiency.